Sound recording



Nov. 14, 1939. J. H. MCLEOD ET AL SOUND RECORDING Filed 0012. 15, 1937 John /7. Mel eod Fre der/bk M 6 Ho/m as I N VEN TORJ ATTORNEYS .7 Patented Nov. 14, 1939 SOUND RECORDING John H. McLeod and Frederick'M. Holmes, Rochester, N. Y., assignors to Eastman Kodak CompanyuRochester, N. Y., a corporation of New Jersey Application October 13, 1937, Serial No. 168,778

only a single track is recorded which can be used, however, for push-pull reproduction. Incidentally, by masking a narrow central portion of the 15 track, it may be made to appear as two separate tracks, but is actually just a single track or at least a track recorded from a single beam of light.

It is the primary object of the invention to provide a simple ribbon valve circuit capable of providing such a track.

20 According to the invention a ribbon valve is provided having two parallel ribbons in a strong magnetic field, each ribbon arranged parallel and under tension in the usual manner. The two ribbons, according to the invention, are insulated 25 from each other and each is separately connected through a half-wave rectifier across an input circuit carrying the signal to be recorded or a portion thereof. The advantages of half-wave rectifiers in this connection are disclosed in a copending application filed concurrently herewith Serial Number 168,777, now Patent No. 2,161,369, to McLeod (one of the present inventors) and Morse. The present arrangement accomplishes somewhat the same result as therein disclosed, but in a much 35 simplified manner. It is, however, confined to variable area recordings. I

The rectifiers are arranged to permit current to flow only in the direction (determined with respect to the magnetic field) which causes each 40 ribbon to move in a lateral direction away from the other ribbon. Thus the't'worectifiers must be oppositely connected.

The invention will be more clearly understood from the following description when read in con- 45 nection with the accompanying drawing in 5 the input 12 of a transformer having a secondary In the preferred embodiment of the invention l3 connected to supply current to two single ribbon light valves l6 and I1. These ribbons actually are mounted in a single light valve but are electrically insulated from each other except for the external circuit as shown. A half-wave rectifier I8 in series with the ribbon [6 permits only one half (e. 'g., positive half) of the input signal to pass through-this ribbon it. The magnetic field of the valve is such that this current causes the ribbon It to move away from the ribbon H. A rectifier IS in series with the ribbon l1 similarly permits it to move only in a direction away from the ribbon l6.

Each of the ribbons may have a separate secondary or may be connected across only a portion of the secondary 13. The simple arrangement shown appears to be preferable, however.

An optical system comprising a lamp 20, lenses 2! and H, slit diaphragms 22, and a cylindrical lens 23, projects an image of the space between the two ribbons to a sensitive film 24 falling on the portion 25 intended for the sound record.

The usual picture area is indicated as 26.

When there is no input signal the points [4 and I5 at the end of the secondary [3 are at the same potential and hence both ribbons remain stationary and are adjusted to be very close together. The image of the space between the ribbons as focused by the lenses 2| and 23 on the film 24 then appears as a narrow band having a width So. On one half of each cycle of any input signal the point 14 is positive with respect to the point 15 and hence a current flows through the valve ribbon I1 and rectifier l9, but is prevented from flowing through the valve ribbon l6 by the rectifier I8. The usual magnetic field is oriented so that this current causes the ribbon I! to move away from the ribbon 16 which remains in its zero position throughout this half of the input cycle. The appearance of the recorded sound track is indicated on the drawing.

It will be noted that the sound record either in the negative or in the positive appears as a single track running lengthwise of the film, more or less centrally located on the area provided therefor and having a width which remains constant when no signal is being recorded, but whose edges vary in accordance with a half wave rectification of the sound wave, in an outward direction only and alternately so that when one edge is varying the other edge remains constant.

Although the present invention is not primarily concerned with galvanometer type recording valves, Fig. 3 is included to show how a track similar to that shown in Fig. 2 could be made with a galvanometer system. A lamp projects light through a lens 3|, a mask 32 having a distorted hour-glass shaped aperture 33 and through a lense 34 to the mirror 35 of a galvanometer. An image of the mask 32 is projected on a slit 3'! in a mask 36. This image 38 moves up and down when the mirror 35 vibrates in accordance with the sound modulations. The width of the band 39 of light formed by this image 38 and the slit 31 is focused by an optical system 40 on a film, not shown, to form a track having a form similar to that shown in Figs. 1 and 2.

Considering again the invention as shown in Fig. 1, it will be noted that a mask placed between the ribbons or at any real image of this area such as at the film 24 will cause the central band So to appear light in the negative and dark in the positive. Such an arrangement would tend to decrease the transmission of the positive track for low volume signal and. hence would be perhaps even more noiseless than the arrangement shown. However, the advantages gained in this direction would be small since class B recordings are, of course, noiseless to a considerable degree. The arrangement shown is, therefore, preferable because of its simplicity.

What we claim and wish to secure by Letters Patent of the United States is:

An electric circuit for noiseless push-pull sound recording comprising an input circuit, a light valve having two single ribbons in a magnetic field, means connecting each ribbon across a portion of the input circuit and a half-wave rectifier in series with each ribbon, said rectifier for one ribbon having a direction of action opposite to that of the other rectifier and such with respect to the magnetic field that a current passing therethrough causes the ribbon to move laterally away from the other ribbon.

JOHN H. McLEOD. FREDERICK M. E. HOLMES. 

